The present invention relates to a method for improving the back contact layer, or more precisely the back contact sequence of layers for CdTe thin-layer solar cells.
In the prior art, CdTe thin-layer solar cells in the superstrate configuration are produced by applying a transparent front contact layer (TCO—transparent conductive oxide) to a substrate (preferably glass). A layer of cadmium sulphide (CdS) is deposited on it and a layer of cadmium telluride (CdTe) is applied over it. The back contact layer or back contact sequence of layers is in turn deposited on the CdTe.
The problem in this case is that it is not possible to apply a metallic contact layer directly onto the CdTe, because this could give rise to the formation of a rectifying Schottky contact. However, the formation of an ohmic contact is what is wanted. The intention of the sequence of layers is to equalize the energy levels of the individual layer materials in a manner such as to form an ohmic contact, wherein the uppermost back contact layer is preferably a metallic layer.
During the manufacturing process, activation of the CdTe by means of CdCl2 and heating are preferably carried out after application of the CdTe.
In prior art methods, the cadmium telluride layer then undergoes a wet chemical etch. To this end, the CdTe solar cell is immersed in a so-called NP etch. The NP etch is an aqueous solution of various inorganic acids, preferably HNO3 (0.5%-5%)/H3PO4 (50%-85%)/H2O (15%-45%) (total 100%). This is carried out within a temperature range from ambient temperature (18° C.) to approximately 80° C. The etching period is preferably in the range 5 s to 60 s. As a result of NP etching, a Te-rich layer is formed which has a thickness in the range 1 nm to 300 nm.
Alternatively, bromomethanol is used as the etching agent. The concentration in this case is preferably in the range 0.1%-5% Br2, particularly preferably in the range 0.5%-1%. During the etching procedure (etching period 3 s to 30 s), a temperature of 15° C. to 50° C., particularly preferably 20° C. to 35° C. and more particularly preferably 21° C. to 30° C. is maintained. The thickness obtained for the Te-rich layer is in the range 1 nm to 30 nm.
The etching step cleans oxides, inter alia, which are formed during the course of the manufacturing process upon contact with the atmosphere, off the surface. Optional additional treatments with reducing solutions may be necessary for this purpose in order to remove the oxides from the Te layer as comprehensively as possible.
Subsequently, in prior art processes, a layer of Sb2Te3 is often applied, preferably by sputtering of Sb2Te3. Next, further plies of the back contact sequence of layers, typically of molybdenum and nickel, are applied. A corresponding process has been described, for example, in U.S. Pat. No. 7,211,462 B1, which dispenses with an etching step and only carries out a CdCl2 treatment of the surface. The Sb2Te3 layer is applied by sputtering in this case. Other known methods for applying the Sb2Te3 layer are vapour deposition or electrolytic deposition. If necessary, a plurality of Sb2Te3 layers may also be provided.
Depositing the Sb2Te3 layer is highly cost-intensive, since Sb2Te3 is very expensive.
Thus, there is a need for the Sb2Te3 layer to be produced in an alternative manner or to replace it with other materials.
This need is fulfilled by the method in accordance with the invention as claimed in claim 1. Advantageous embodiments are disclosed in the dependent claims.